Described herein are compounds having the general formula: ##STR1## Ar.sub.1 and Ar.sub.2 are independently selected aryl groups, optionally substituted with substituents selected from the group consisting of alkyl, cycloalkyl, alkoxy, alkanoyl, aralkyl, aralkyloxy, halo, NO.sub.2, Ph, CF.sub.3, CN, OH, methylenedioxy, ethylenedioxy, SO.sub.2 NRR', NRR', CO.sub.2 R (where R and R' are independently selected from the group consisting of H and alkyl) and a second aryl group, which may be substituted as above;wherein any cycloalkyl or aryl substituent is linked to Ar.sub.1 or Ar.sub.2 by a bridging element which may be a single bond, a vinylene group, an ethynylene group, a Z group, a --Z--(CH.sub.2).sub.n -- group, a --(CH.sub.2).sub.n --Z-- group, or a --Z--(CH.sub.2).sub.n --Z-- group, where Z represents an O atom, a S atom, an NH group or an N-alkyl group, and n is an integer from 1 to 4;wherein Ar.sub.1 and Ar.sub.2 may be attached to the central atom to which they are connected by a single bond, an alkylene, alkenylene or alkynylene group;R is H, alkyl or the counter ion for a basic addition salt;R.sub.1 and R.sub.2 are independently selected from the group consisting of H, alkyl and benzyl;R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are independently selected from the group consisting of H and alkyl;and a stereoisomer, salt, solvate and hydrate thereof.Also described is the use of these compounds as pharmaceuticals for the treatment of diseases in which inhibition of glycine transport via the GlyT-2 transporter is indicated.